Phenol is manufactured via air oxidation of cumene to cumene hydroperoxide (CHP), followed by acid-catalyzed cleavage of the latter to phenol and acetone. CHP decomposition is a very exothermic reaction which is normally carried out on a commercial scale in continuous stirred or back-mixed reactors. In such reactors only a small fraction of CHP is unreacted any given time and the reaction medium consists essentially of the products of decomposition of CHP, i.e., phenol and acetone, plus any solvent (e.g., cumene) and other materials added with CHP to the reactor. During cumene oxidation small amounts of dimethyl phenyl carbinol (DMPC) and acetophenone are also formed. In the presence of acid catalyst, DMPC dehydrates to alpha-methylstyrene (AMS), a useful by-product. Very high yields of AMS can be obtained from pure DMPC, e.g., 98% yield upon dehydration over acidic silica at 300.degree. C. In the presence of phenol, however, and more specifically in phenol/acetone/cumene which is solvent in decomposition of technical CHP/DMPC mixtures, the ultimate AMS yield is normally about 50-60 mol% of the DMPC. Main by-products are AMS dimers and cumylphenol which have no commercial value. Formation of cumylphenol also reduces the phenol yield.
G. G. Joris, U.S. Pat. No. 2,757,209, teaches that the amount of AMS dimers and cumylphenol formed can be substantially reduced by carrying out the reaction in two stages. In the first stage CHP is decomposed in a stirred or back-mixed reactor in the presence of small amounts of sulfur dioxide as catalyst and water as catalyst moderator. Preferred conditions are: temperature 45.degree.-65.degree. C., sulfur dioxide 50-500 ppm, water 2-5 wt%. Under these conditions the CHP concentration in the reaction mixture withdrawn from the reactor is less than 5% but more than 1% by weight. In the second stage, the mixture withdrawn from the first reactor is heated in a second reactor, optionally with additional catalyst, in order to effect the dehydration of DMPC to AMS. This second reactor is either a batch reactor, or a continuous plug-flow reactor. Preferred conditions are: temperature 110.degree.-120.degree. C., reaction time 5-15 min. Care must be taken to stop the high temperature reaction once AMS formation is completed so as to avoid the dimerization of AMS or the reaction of AMS with phenol to form byproducts.
C. Y. Yeh, et al, U.S. Pat. No. 4,016,213, teaches a modification of the above process wherein the back-mix reactor is operated in a manner to avoid the dehydration of DMPC. The mixture withdrawn from the back-mix reactor is immediately treated with base and the DMPC is subsequently recovered from the final distillation wherein crude phenol is the overheads and DMPC is found with acetophenone in the bottoms.
H. Boardman, U.S. Pat. No. 2,668,180, teaches that DMPC and CHP interact in the presence of an acid condensation catalyst to form dicumyl peroxide (DCP). The reaction medium is excess DMPC; examples of catalysts used are p-toluenesulfonic acid, sulfuric acid and boron trifluoride.
Ko. Tsunoda and T. Kato, Nippon Kagaku Zasshi, Volume 81, No. 2, page 310 (1960) studied the reaction of DMPC with CHP in homogeneous benzene/acetic acid solution using perchloric acid as catalyst and also in a two phase benzene/aqueous mixture using sulfuric acid as catalyst. They report that two reactions take place simultaneously: (a) condensation of DMPC with CHP to form DCP, and (b) decomposition of CHP to phenol and acetone.
These last two references indicate that DCP may also be formed during the acid catalyzed decomposition of technical CHP. M. S. Kharasch et al, Journal of Organic Chemistry, Volume 15, page 753 (1950), reported that DCP is decomposed in acetic acid solution in the presence of catalytic amounts of perchloric acid to phenol, acetone and AMS dimer. It is, therefore, possible that DCP is formed during the decomposition of technical CHP and that at least some of the AMS dimers formed during the process originate from subsequent decomposition of DCP.